riscv: Drop USE_SPL_FIT_GENERATOR

Now that we have switched to binman to generate u-boot.itb for all
RISC-V boards, USE_SPL_FIT_GENERATOR is no longer needed and can
be dropped.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>